1. Field of the Invention
The present invention relates to a heating unit used in e.g. a printer for heating printing paper to thermally fix toner on the printing paper. In particular, it relates to a heating unit whose substrate is made of a ceramic material such as aluminum nitride (AlN). The present invention also relates to a method of making such a heating unit.
2. Description of the Related Art
For thermally fixing toner on the surface of a printing paper in a printing process, generally, a toner image formed on the surface of a photosensitive drum is transferred onto the printing paper, and then the printing paper is heated by a heating unit to fix the toner on the printing paper with the heat provided by the heating unit. Such fixing process is normally executed while conveying the printing paper under pressure through between the heating unit located on the side of the back surface of the printing paper and a pressure roller located on the side of the main surface of the printing paper. To efficiently fix the toner while quickly conveying the printing paper, it is effective to expand the area that can be heated by the heating unit, in other words the heating width along the conveying direction of the printing paper, as much as possible.
FIG. 7 depicts a heating unit designed from such viewpoint (for example, disclosed in JP-A-2002-75599). The heating unit X is a strip-shaped plate elongated in a direction perpendicular to the paper conveying direction. The heating unit X serves to heat the printing paper P, on which the toner T has been transferred, held under pressure provided by the pressure roller R, to fix the toner T on the printing paper P.
The heating unit X shown in FIG. 7 includes a ceramic substrate 101 made mainly of aluminum nitride (AlN), oxide layers 102 covering the main surface 101a and the back surface 101b of the AlN substrate, a heat-generating resistor 103 constituted of silver and palladium and formed on the back surface 101b, and a cover layer 104 printed in a form of a thick film to cover the heat-generating resistor 103. In FIG. 7, the AlN substrate 101 is oriented such that the main surface 101a is located on the upper side in the drawing, and the back surface 101b on the lower side. The cover-layer 104 includes a first cover layer 141 formed to cover the heat-generating resistor 103, and a second cover layer 142 formed to cover the first cover layer 141. The oxide layer 102 is formed as a result of oxidation of the main surface 101a and the back surface 101b of the AlN substrate 101, through the sintering process of the heat-generating resistor 103. The first cover layer 141 is formed of crystallized glass, and the second cover layer 142 is formed of non-crystalline glass. Also, although not shown, the back surface 101b of the AlN substrate 101 includes an electrode layer for supplying power to the heat-generating resistor 103.
In the heating unit X, when power is supplied to the heat-generating resistor 103 via the electrode layer which not shown, the heat-generating resistor 103 generates heat at a predetermined calorific value. The AlN substrate employed in the heating unit X is highly heat-conductive, and hence the heat is efficiently transmitted throughout the entire substrate. Accordingly, locating the heat-generating resistor 103 on the back surface 101b of the AlN substrate 101 and providing the printing paper P on the main surface 101a as shown in FIG. 7 allows the overall main surface 101a of the AlN substrate 101 to act as a heating surface, thereby efficiently heating the printing paper P. Also, since the heat spreads all over the AlN substrate 101, the AlN substrate 101 can be kept from cracking or being otherwise damaged because of an internal temperature difference.
When manufacturing the heating unit X thus configured, glass paste is print-sintered after sintering the heat-generating resistor 103, to thereby sequentially form the first cover layer 141 and the second cover layer 142. Upon print-sintering the glass paste on the AlN substrate 101, oxygen in the glass component and nitrogen in the AlN substrate 101 are reacted, thereby foaming. Accordingly, in the heating unit X, the crystallized glass, which generally has a porous structure is utilized as the first cover layer 141, to discharge the foam quickly.
Recently, however, the printing apparatus has also come to be required to incorporate a measure against a lightning surge, and the components incorporated in the printing apparatus such as the heating unit X are required to have a still higher withstand voltage. Although not shown, the second cover layer 142 of the heating unit X is also provided with a thermistor that controls the heating unit X to facilitate the printing paper P to pass on the main surface 101a of the AlN substrate 101, as well as a thermoswitch and a thermal fuse for disconnecting the power when the control is disabled for some reason. The thermistor, thermoswitch and thermal fuse generally include metallic parts. Such metallic parts may serve as the ground, such that when a transitional surge emerges in the heat-generating resistor 103 from switching or lightning, the first cover layer 141 and the second cover layer 142 suffer a dielectric breakdown. Since the heating unit X employs the crystallized glass which often has a porous structure as the first cover layer 141, sufficient insulation performance cannot be expected, and therefore the surge issue is particularly critical.